The present invention relates to a screw-rotating/injection device of an injection molding machine, particularly to a screw-rotating/injection device comprising two electric injection servomotors arranged on a movable pressure plate to rotate a ball screw and a screw shaft such that the length of the device can be reduced, and further comprising a force sensor to detect the pressure of molten material so that a close-loop control for the holding pressure or back pressure of the screw shaft can be accomplished in a injection molding machine to facilitate precise injection of molding processing.
As shown in FIGS. 1a through 1d, a conventional injection molding process generally comprises the steps such as closing and locking a mold, injecting molding material into the mold and holding molding pressure within the mold, cooling and measuring the molding material, and opening the mold and ejecting the molded product. Plastic molding material stored in a hopper (A) can be fed into a heating cylinder (B) by gravity. When a screw shaft (C) is rotated by a hydraulic motor (H), the molding material can be mixed and transferred to the front end of the heating cylinder (B) along a screw groove within the heating cylinder. At the same time, the molding material passing through the heating cylinder (B) is molten by a heating source around the circumference of the heating cylinder (B) and the shearing effect caused by the rotation of the screw shaft (C). When the molten molding material is transferred to the front end of the heating cylinder (B), a counter-pressure will be generated to force the screw shaft (C) back to a predetermined position (a measuring point), and then stops the rotation of the screw shaft (C). Then, the injecting device of a hydraulic cylinder (D) (i.e., an injection cylinder) drives the screw shaft (C) to move forwardly, such that the screw shaft be served as an injection plunger to inject the molten molding material at the front end of the heating cylinder (B) through a nozzle (E) to a closed and locked mold (F) with high pressure and high speed. When a molded product (G) made from the molding plastic material is cooled at a predetermined holding pressure, it can then be ejected from the mold when the mold (F) is opened. Then, the mold can be re-closed to perform the next cycle of molding operation. As mentioned hereinabove, it can be seen that the screw shaft (C) should have the capability for rotational motion and reciprocatively rectilinear motion which are respectively driven by the hydraulic motor (H) and the injection hydraulic cylinder (D). However, such a hydraulic driving system is not only difficult to install and maintain but also energy consuming. In addition, the position control characteristic thereof is poor and the hydraulic pump thereof will generate noise, vibration and oil vapor to deteriorate the working environment.
In view of this, U.S. Pat. No. 4,693,676, the disclosure of which is incorporated by reference herein, and entitled to Fanuc Ltd., Minamitsuru Japan, disclosed a screw-rotating/injection mechanism of injection molding machine which utilizes servomotors to rotate the screw shaft and to drive the injection operation. As shown in FIG. 2, such a screw-rotating/injection mechanism comprises a servomotor (M1) for driving a driving wheel (13'). The screw shaft (1') thereof can have a measuring rotation by the driving of the driving wheel (13') through the rotation transmission of a timing belt (15'), driven wheels (9'), a shaft (10'), an outer spline shaft (5') and a connecting shaft (1"). A servomotor (M2) rotates a driving wheel (14'), through the torque transmission of a timing belt (16') and driven wheels (11', 11"), so as to rotate the ball screws (8', 8") and drive the ball nuts (7', 7") and pressure plate (4') to move in order to accomplish the processing of injection, holding pressure and backing pressure. Though the using of such a servomotor can avoid some defects of conventional hydraulic injection devices, there are still some defects remained to be improved. For example, when the spline shaft (5') slides in the spline groove (10'a) to perform a measuring rotation of the screw shaft, the torque transmitted between the spline shaft (5') and groove (10'a) is very large and the frictional wearing between the spline shaft (5') and the spline groove (10'a) is unavoidable and thus will deteriorate the precision of operation and decrease the duration of the device. Besides, the sliding arrangement of the spline shaft and the spline groove takes much room rendering the molding machine is lengthened and bulky. When a plurality of ball screws (8', 8") are driven by an injection servomotor (M2), the pressure plate (4') tends to be oblique since ball screws are exerted unequal force or do not move in synchronism due to the different lead error between the ball screws and the backlash between the belt and wheel. As a result, the screw shaft (1') and the heating cylinder (2') tend to be worn out, and influence the quality of the molded products and the stability thereof. In addition, the steel balls of the ball screws are easier to be broken. Further, such a device does not include any feedback sensor, it therefore cannot complete a close-loop control for holding pressure and back pressure, and thus disadvantage the precise injection molding processing.